German laid-open patent application DE 102 11 948 A1 has disclosed a computed tomography unit that is equipped with an X-ray tube with a fixed focus and has, situated opposite, a multirow detector that is equipped in a first variant (FIG. 3) with a so-called z-comb that acts as an aperture stop in the z-direction for the detector rows. In a second variant (FIG. 4), the laid-open patent application discloses a detector diaphragm that, in addition to the aperture stop in the z-direction, also has an aperture stop in the circumferential direction or longitudinal direction of the detector. Thus, the active cross sections of the individual detector elements are constricted from all sides such that their aperture is reduced in two dimensions.
Fundamentally, such a reduction in the active cross-sectional surface takes place in order to improve the resolution of the detector. However, a reduction in the cross-sectional surface is simultaneously attended by a reduction in the sensitivity of the detector and leads—at least in the case of human diagnosis—to an undesired increase in the radiation burden such that limits are set for the reduction in the aperture by the reduction in the sensitivity of the detector.